It is generally known to those skilled in the art that ferrites absorb microwave such as those of 500 MHz to 12 GHz to change the microwave energy to a thermal energy. The ferrite is a sintered body having the spinel structure and it is a compound having the following general formula: EQU MFe.sub.2 O.sub.4
(wherein M is a divalent metal such as Mn, Ni, Cu, Zn, Mg, Co, etc.).
The use of a sintered body of the ferrite for preventing the leakage of microwave in "microwave heating oven" has been disclosed in U.S. Pat. No. 2,830,162. The microwave heating oven is a device for heating a material with microwaves generated by a microwave generating device such as magnetron. The microwave, however, can be absorbed more effectively by the powder of ferrite (ferrite powder) than the sintered body of ferrite. A mixture mainly composed of ferrite powder for use in absorbing the microwave has been disclosed in U.S. Pat. No. 3,742,176. In this patent, a mixture of ferrite powder and an insulating material such as rubber has been disclosed. The invention of this (U.S. Pat. No. 3,742,176) has been accomplished by the same inventor as one of the inventors of the present invention. The inventors of the present invention found that the effect of absorption of the microwave depends on both the frequency of microwave and the particle size of ferrite powder. Namely, the ferrite powder having a certain limited particle size can absorb more effectively microwave having a certain frequency. The present invention is based on the findings as set forth above.